1. Field of the Invention
The present invention relates to thermoplastic polyurethanes which have significantly improved abrasion resistance while retaining other desirable mechanical properties.
2. Description of the Prior Art
Preparation of thermoplastic polyurethane materials is well known in the art. Such materials can be extruded, injection molded and fabricated in other known manners to form many different shapes and configurations which find utility in a large number of applications.
Thermoplastic polyurethane materials can be prepared by reacting polymeric polyols, diisocyanate compounds and polyfunctional chain extender compounds having active-hydrogen groups such as hydroxyl, primary amino, secondary amino, and mixtures thereof. Various combinations of polyester polyols and polyether polyols have been employed in the preparation of polyurethane materials having a diversity of properties.
U.S. Pat. No. 3,493,634 discloses thermoplastic polyurethane elastomers prepared from a mixture of a polyester diol, polyoxyalkylene glycol and glycol extender.
U.S. Pat. No. 3,798,200 and 3,963,681 disclose polyether mixtures in the preparation of soft crosslinked polyurethanes.
U.S. Pat. No. 4,124,572 discloses blends of polyester polyols with particular polyoxypropylene-polyoxyethylene glycols which are said to have higher tensile strengths than prior art polyether and polyester based polyurethanes.
U.S. Pat. No 4,182,898 discloses the use of isocyanate terminated prepolymers prepared from a blend of polyester polyol and polyether polyol in the preparation of cast polyurethane elastomers.
U.S. Pat. No. 4,379,904 discloses a thermoplastic polyurethane elastomer having improved impact properties prepared from 4,4-methylenebis(phenyl isocyanate), difunctional extender and particular polyoxypropylene-polyoxyethylene copolymers and/or polyester diols.
British Patent No. 1,233,614 discloses polyurethanes having improved stiffness-temperature properties prepared by reacting an organic polyisocyanate with a mixture containing particular incompatible low and high molecular weight polyols.
British Patent No. 1,410,809 and its equivalent German Offenlegungsschrift No. 2,261,482 disclose a polyurethane elastomer prepared from a polyfunctional isocyanate, a curing agent and a mixture of polyethers having active-hydrogen terminal groups, wherein the weight average molecular weight of the polyether mixture is between 4,500 to 20,000 and the molecular weight distribution curve of the polyether mixture has at least two peaks. These polyurethane elastomers are alleged to have good cut growth and flex crack resistance properties.
British Patent No. 1,388,748 and 1,389,039 disclose a polyester-polyurethane product prepared by reacting a polymeric diol mixture comprising (a) 3-30 percent by weight of a high molecular weight poly(alkylene alkanedioate) glycol or poly(oxycaproyl) diol and (b) 97-70 percent by weight of a low molecular weight poly(alkylene alkanedioate) glycol or poly(oxycaproyl) diol and which polymeric diols differ in the molecular weight by at least 1,000 with an organic diisocyanate and a difunctional chain extender. These materials are said to be better adapted for end uses such as in shoe soles and solid truck tires due to their improved split tear strength and elongation properties.
British Patent No. 1,155,548 discloses polyurethane materials particularly adapted to injection molding and having a good balance of physical properties. These polyurethanes are particularly designed to overcome the problems encountered by prior art materials in injections molding such as difficulty in filling mold cavities, sticking in the mold, shrinkage and long cycle times. These polyurethanes are prepared by reacting a mixture of 60-85 parts of a hydroxyl terminated existentially linear polyester having a molecular weight between 900 and 3,000, 15-40 parts of an essentially linear hydroxyl poly(alkylene oxide) having a molecular weight between 800 and 2,000 and 1.25-12.8 mols of an aliphatic glycol per mol of total hydroxyl polyester and hydroxyl poly(alkylene oxide) with an aryl isocyanate in a molar amount equal to the total mols of hydroxyl polyester, hydroxyl poly(alkylene oxide) and aliphatic glycol.
Thermoplastic polyurethane materials have become of increasing importance for use in manufacturing shoe soles, sport shoe soles and particularly ski boots. As demand has risen for such materials so have the physical property requirements increased, particularly requirements for resistance to high abrasive and destructive forces to which ski boots and sport shoe soles are constantly subjected to. Although the prior art is capable of producing polyurethane materials which can be used in these applications, there is still great need and demand for materials which will have considerably improved resistance to abrasion forces than the prior art material.
It has now been discovered that a thermoplastic polyurethane can be prepared which exhibits surprisingly high resistance to abrasion forces while retaining a spectrum of other desirable chemical properties including high impact resistance and creep behavior. This high resistance to abrasion forces is quite unexpected in view of the performance characteristics of similar prior art materials. The new thermoplastic polyurethanes are particularly useful in the manufacture of shoe soles, sport shoe soles, ski boots and bindings, various automotive uses such as car bumpers and side guards, conveyor belts and screens for use by mining industry, oil seals and the like.